To extend your tourist permit in Santa Marta if your 90 days has expired you first need to make an appointment at Migracion in Santa Marta. You do this by going online and following this link https://www.migracioncolombia.gov.co/agendarsucita. Fill in your details and select the appointment date. Fill out the form without using the automatic translate as the website has many bugs. You will receive and email confirming the date.
After this you will need to go to this link and fill out what they call the FUT https://apps.migracioncolombia.gov.co/registro/public/formularioRegistro.jsf. You will need to select this option Permiso Temporal de Permanencia para Prorrogar Permanencia. Again fill out the form without using the automatic translation. You will need to upload a copy of your passport in jpg so make sure you have that ready.
When you go to Migracion in Santa Marta take along photocopies of your passport, FUT and the appointment confirmation. Arrve 15 minutes early and speak to the guard on the door. Take a bank or credit card to pay as they do not take cash. If you pay in cash you will need to go to Banco Occidente in the centre and queue for probably over an hour and then return to migracion to finish the process. The cost of this process is 99000 pesos.
The address for Migracion Santa Marta is Calle 19 with Carrera 9 ‘Avenida del Ferrocarril